Test for multiple expressions feature.
Implemented support for specifying multiple expressions.

Each expression works on a line modified by the previous one, just like in sed.

This also changes the error message (and fixes its test) so that it contains
the expression which caused the error, to make it easier to understand when
multiple expressions are used.
Added note about multiple expressions to README.
Fixed: unit tests necessarily running right version of Python.

Without this fix, the tests which actually execute pyle.py would do so using whatever `python` was on the path. But the tests might have been run with a special interpreter, like that from inside of a virtualenv, without the path being modified. Now the tests would be run with an interpreter not meant to be tested.

This fix makes sure to use the same interpreter as the one running the tests.
Merge pull request #1 from jitakizushi/multiple_expressions

Multiple expressions

Conflicts:
	pyle_test.py
Fixed: some README typos.
Bump version to 0.3.