Numbertoletters

Fixed missing white space after 'millones' format

Fixed typos and Improve base code

Strip resulting numbers.

**Before**

python
number_to_letter(1235)
>>> 'mil doscientos treinta y cinco '


**After**

python
number_to_letter(1235)
>>> 'mil doscientos treinta y cinco'

First release of the library.

Includes these fixes:
- Negative support
- Handle `zero` properly