Merge pull request #10 from l-sora/master

Adding missing features:
Added some test cases to regex -> nfa
Fixed test failing test cases
Merge pull request #11 from omar-ocampo/foocampo_dev

Added some test cases to regex -> nfa
When receiving a string of size 0 the method 'infix_to_prefix' will
return a string representing the empty string: '#'. This is needed because
internally we handle the empty string as '#' and not as ''.
Make the window unresizable and also make the initial position the center
of the screen.

Placing the window on the center makes the window flash for a brief moment
in the wrong positio. This is due to the methods available in TkInter.
Modified the GUI layout. Added new button to erase all the fields.
Add keyboard binding so when the user presses enter the comparison of both
regular expressions is done.