creating is_valid method as suggested in the issue 134

[sjortiz]

Originated from the issue #134

Description:
This methods returns True if the given data has passed all the validations that were specified in the given schema, returns false if not.

Considerations:

This method doesn't raise any exception and thus prevent any exception from the schema validation to pass silently

[codecov-io]

Codecov Report

Merging #150 into master will decrease coverage by 2.33%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage     100%   97.66%   -2.34%     
==========================================
  Files           1        1              
  Lines         208      214       +6     
==========================================
+ Hits          208      209       +1     
- Misses          0        5       +5

Impacted Files	Coverage Δ
schema.py	97.66% <16.66%> (-2.34%)	⬇️

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[skorokithakis]

Thank you!

[mlhamel]

❤️