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.

When a user requests a specific todo ID, always show it, even when the
todo item is not relevant.

Conflicts:
	topydo/commands/ListCommand.py

Conflicts:
	setup.py

Also marked a few functions private.

Move format parsing logic to ListFormatParser

This way we can safely remove `PrettyPrinterIndentFilter` and output
length is still properly maintained by `ListFormatParser`.

ListFormatParser is now the core of PrettyPrinterFormatFilter, but can
also be easily used in another areas. Whole functionality of
PrettyPrinterAlignFilter was also moved into it.

Main advantage of having separate parser for list_format is possibility
of initializing it and resolving placeholders only once. Previous
implementation did that with each Todo object.

- From now on literal `%` signs always have to be escaped with `\` (`\%`),
even inside conditional braces.
- `%%` won't work anymore.
- Nonexistent placeholders (for example`%&`) will be expanded to empty
  strings (`''`).

This change fixes also some bugs:
- inappropriate output width when \t was used without %S
  (example: `%p{ }\t%{ }p`)
- curly braces inside conditional braces (example: `%{{}p{}}`)

Minor: recognize literal `\t` in list_format config option and in `-F`.

They mostly deal with corner cases (small terminals, dodgy texts in the
prefix/suffix).

Also prepare the test for the new syntax where a literal % will be
written as \% instead of %%.

This corresponds to the change made in commit 33794435.

Update tests for ls_format

Test ignore_weekends clause in Importance

2015-11-06 as freeze_time date was introduced in #75 so let's stick with
it also here.

Also introduce freezegun.freeze_time decorator.

Update tests for Importance

2 more tests of calculation importance from due date.

This increased (branch) coverage to 100%. A few corner cases were
missing with strange default commands in the configuration.

This was missing according to coverage.