[security] introduce sensitve value object.
add more tests. fix typo in test file name.
[security] sensitive value object add peek method and change meaing of get method.
[security] add docs for sensitive values.
[security] make sure doctrine storage work with sensitve value and does not store it.
Merge branch 'storage-filesystem-add-identity-map' into security-introduce-sensitive-value-object
apply changes according to cordoval's comments
[doc] reflect in the doc that sensitive value does not encoded to json.
Merge pull request #79 from Payum/security-introduce-sensitive-value-object

[WIP][security] introduce sensitive value object.