Add partial diff/patch functionality.

This commit adds barebones capacity to generate diffs from two trees and
to emit those as git-style diffs (via `Patch.String`), or to enumerate
the files/hunks/lines in the diff to emit the data yourself.

The walk functions have been implemented in the same manner as the Odb
walking methods.

Note that not all of the functionality is implemented for either the
`git_diff_*` nor the `git_patch_*` functions, and there are unexposed
constants which would likely be useful to add.
Export the constants and enumerations for diffs.

It is possible that the typed enums from libgit2 should be exported as
distinct types rather than numeric constants (e.g, `git_delta_t` and
`git_line_t` should be typed), but this has not been done in this patch.
Pre-emptively copy data when marshalling diff callback data.

When marshalling diff callback data to Go structs, any `char*` need to
be pre-emptively copied onto Go's heap as they're invalidated as soon as
our callback function returns. This patch adds this extra copy before
sending the value to the channel, which fixes a bug wherein
`DiffLine.Content`, `DiffFile.Path` and `DiffHunk.Header` would
previously return garbage data.
Unwrap DiffLine; add types for git_diff_flag_t and git_delta_t.
Bundle consts more idiomatically; add GitLineType alias.
Actually type constants; unwrap DiffFile, DiffDelta, DiffHunk.
cleanup and refactor diff / patch
merge with latest
refactor and cleanup code
set ptr to nil after free
Allow diff.ForEach to enumerate files, hunks, and lines with single call. Support use of closures for enumeration.
don't expose 3 different diff foreach methods. use structures instead of pointers to structures for diff detail. add patch error code handling. trim excess data from diff structures.
fix go 1.0 compile error
Merge remote-tracking branch 'libgit/master' into add-basic-diff-patch
use cast_ptr instead of ptr for travis go tip build
Merge commit 'refs/pull/72/head' of github.com:libgit2/git2go

Conflicts:
	git.go
	wrapper.c