Update endians.nim

adds swapEndian16 function
Adds example to locals() docstring.
Adds docstring to rstgen.defaultConfig.
Update os.nim

added file/dirExists synonyms for existsFile/dir
Added --noBabelPath override.
Define $ operator for TRune
Define $ operator for TRune
Remove assert in asyncio which causes problems.

This assert keeps failing occassionally in nimbuild causing it to crash.
According to logic it shouldn't, but perhaps the socket's writeable
status changes.
Fixes #594

This was already partially fixed, ``expandTilde`` was the missing piece
from perfection.
httpclient: strip trailing whitespace from headers.

- we can do that according to the RFC.
- fixes #783
Merge pull request #776 from gradha/pr_docs

Docstrings
Removes generated .dot file uploaded by mistake.
Merge pull request #791 from gradha/pr_removes_generated_file

Removes generated .dot file uploaded by mistake.
Merge pull request #788 from zielmicha/httpclient-fix

httpclient: strip trailing whitespace from headers.
Added docgen documentation
Fixes two minor typos.
Reformats text to fit width of 80 columns.
Adds docgen to list of documentation to build.
Adds some cross references to docgen manual.
Transforms docgen sample to be generated from source.