(try to) Improve the user Manual

- split install section intro "prerequisite" and "installation"
- specify that gcc>=4.7 is required
Uses string.startswith('...') instead of a regexp

Aka the right tool for the right job :)
Remote redundant import
Defensive approach in the pass manager when apply() on unknown pass type

I still don't know if we should raise or warn or whatever here. At least
do not call blindly display() unless there is a super-class for passes
that provide it.
Defensive programmign apart, code is more readable with the check for
'Analysis' type. Still wonder if "transformation" is the right name
for the pass.
Improve comments in syntax module (Pythran Syntax Checker)

Conflicts:
	pythran/syntax.py
Pythran main: read only input file once