Parameterized constructors for StdVector

[omus]

Fixes #367. With this PR the StdVector constructors now support specifying the element type of the vector. Additionally, I've refactored the code to be more inlined with how Vector behaves and have added a more extensive test suite.

Example of the new behaviour of this PR using the examples from #367:

julia> StdVector{StdString}()
0-element CxxWrap.StdLib.StdVectorAllocated{StdString}

julia> StdVector(StdString[])
0-element CxxWrap.StdLib.StdVectorAllocated{StdString}

julia> StdVector(StdString["hello"])
1-element CxxWrap.StdLib.StdVectorAllocated{StdString}:
 "hello"

julia> StdVector{StdString}(StdString["hello"])
1-element CxxWrap.StdLib.StdVectorAllocated{StdString}:
 "hello"

[barche]

This looks good to me, so if it's ready for you I can merge it.

[omus]

Ready to merge. Thanks!