add license information to gemspec

This way you can find this information with the rubygems.org API
Merge pull request #58 from jordimassaguerpla/master

add license information to the gemspec
bump to latest headers, close #63
update travis image
tests
allow set history filename
thanks :)
make contributing policies more known
Bump to version 1.3.2
Bump to version 1.3.3
Add support of different Printers

So now we can specify different printers, which defines the text we send
to the interface. Added support of plain text printer (which displays
the texts currently used in debugger) and XML text printer, which I'm
going to use to recreate ruby-debug-ide functionality here, in the
'debugger' gem

You can specify what printer to use by Debugger.printer accessor.
Add XML texts for the 'break' commands
Add XML output for 'condition' command
Add XML output for the 'continue' command
Add printers support to 'restart' command
Add printers support to the 'display' command
Add printers support to the 'edit' command
Add more printers support for enabling/disabling, deleting breakpoints
Add printers support to the 'eval' command
Add printers support to 'frame' commands

Also, tweaked the Context#frame_id, Context#frame_file and
Context#frame_line functions, so they don't fail with segfaults if
rb_iseq_t *iseq points to nowhere. Unfortunately, my C/Ruby internals
skills are not good enough to understand why *iseq maybe sometimes nil,
but better to not show some frame line sometimes than failing with
segfault.

We probably need to figure out though why *iseq may be nil sometimes