Runtime: When testing for boolean values in can(), test for value equality.

Prior to this can() simply returned the value of the cap, rather than a
result of comparison.
Runtime: Prefix private variables with underscore.
Runtime: Introduce new property - mode.

Runtime can reside in two modes: browser and client (default is
browser). This is mainly introduced for non-native runtimes -
Flash/Silverlight, that may operate in alternative modes.
Runtime: Remove setCap(), since it's more of hack.

Alternatively it should be possible to extend set of capabilities
during the construction process. This way mode will be set only once
and properly reflect the state of runtime's capabilities.
RuntimeClient: Check for runtime mode, instead of capabilities.

Runtime will be tested against required capabilities (if any)
internally, during the construction process.
Runtime: In can() check for equality of value and result for both - properties and methods.
Runtime, HTML5: Make set of capabilities extendable.
Runtime, Flash/Silverlight: Separate all capabilities that depend on the mode.
Tests, Runtime: Add mode tests.
XHR: If withCredentials not set, but assumed in required capabilities, auto-set it.

Other runtimes (Flash/Silverlight) do not require this property, as
they do this by default.
Tests, XHR: Fix test for browser session maintainablity by runtime.