Priority command: allow lowercase priorities to be input

Don't just consider the first p tag value in a todo item, consider all
of them.

Given these dependencies:

A id:1
B p:1
C p:1 id:2
D p:1 p:2

A -> B
A -> C
A -> D
C -> D

The relation A -> D is superfluous (A -> C, C -> D). However, the p:1
tag in D would not be removed, because relation 1 still exists (A -> B).

Therefore a new function is added that identifies p: tags which have no
corresponding edge in the internal dependency graph (rather than looking
at the value of the edge).

Conflicts:
	setup.py
	topydo/lib/PrettyPrinterFilter.py

This assertion was missing in commit 6bb804dc.

To complete a todo item that you finished yesterday, run

    topydo do -d yesterday

Or

    topydo do -d -1d

Inspired by issue #82.

When an ordinal tag filter is negated (i.e. -id:1), then this isn't seen
as a negation but as an ordinal tag filter with tag -id and value 1.
Which isn't likely to exist, resulting in every todo item to be excluded
from the result.

Related to commit cde8f8ed

https://bitbucket.org/ned/coveragepy/issues/90/lambda-expression-confuses-branch

When the due date tag or the start date tag is set, convert the value to
an absolute date if a relative date was given.

An explicit start date is given with the t: tag (unless configured
otherwise). Together with the due date, the start date is used to
calculate the length of a todo item (in days).

When no explicit start date is given, consider the creation date as the
start date (if present).

Improves branch coverage. Also related to commit cde8f8ed.

pypy was still broken in the Graph class, but in this branch this is no longer an issue.

The lambdas, which are not all run, cause coverage to think that the
jump to the function exit was never made, polluting the results.
Therefore add this pragma to mask these instances of missing branch
coverage.

See also:

https://bitbucket.org/ned/coveragepy/issues/90/lambda-expression-confuses-branch

The result of a minor merge error.

This reverts commit bbdfb2a5.

(Wheel) dependencies are evaluated at build time, so they should stay in
extras_require, which are evaluated at install-time.

See issue #79.

Add new tests to increase lib coverage

Mainly adds a missed case in ExpressionCommand, where priority filters were not covered.

Supporting Python 2 resulted in some additional dependencies (six,
ushlex, get_terminal_size). Removing 2.7 support makes things a bit
easier and maintainable.

Installing from the source distribution (sdist), ushlex and
shutil_get_terminal_size were not installed. Putting the condition in
install_requires addresses this.

Oops! Almost forgotten.