Add docs for all available utilities

[reedsa]

Document utils for conversions, decorators, exception and module loading. text_if_str hexstr_if_str, etc.

What was wrong?

We were missing docs for several utility functions.

Closes #139

How was it fixed?

Filled in docs for utilities that were undocumented.

Todo:

• [x] Clean up commit history

• [x] Add or update documentation related to these changes

• [x] Add entry to the release notes

Cute Animal Picture

[fselmo]

@reedsa Looks like even though the docs build, there's a table formatting error after making the spacing more homogenous.

When running make build-docs:

.../eth-utils/docs/utilities.rst:811: ERROR: Malformed table.

We could probably leave that table alone in that commit.