Introduces GlobalChecks, closes #27

[phosphore]

This pull introduces a new class of Checks called GlobalChecks. These non-standard checks are executed after the first round of standard Checks and they work on the array of issues generated by the first batch of Checks to determine further vulnerabilities or discard false positives.
To do this, it is now possible for the standard checks to share an additional properties object when added to the initial issues array, that can later be used by any GlobalCheck to perform decisions. Using this class of checks provides an improved decisional process, which will come in handy in other checks that need to scan every JS/HTML of the target application before determining the presence or absence of a vulnerability (e.g. the Affinity check).
These checks are located in src/finder/checks/ComplexChecks/ and share a very similar loading mechanism to that of the standard checks.

This pull request adds the first GlobalCheck ever, used to check for CSP. See the old PR #35 for more information.

This pull request adds positive/negative tests for HTML/JS CSP checks.

[phosphore]

I created a page for the CSP check in the project wiki.

[phosphore]

Updated the CONTRIBUTING.md to add GlobalChecks.
Renamed the CSPGlobalCheck ID from CSP_CHECK to CSP_GLOBAL_CHECK.
Updated the respective wiki page.

[ikkisoft]

Can we close this one, since you've opened another one?