Add leading space to comments
Safely concatenate addresses.

Formerly, appending directly to e.Bcc might have made unintended changes
to its underlying array.
Tighten Attach and AttachFile
Make Email.Attachments a slice
Return errors in Bytes()
Soup up base64Wrap, add test and benchmark.

It now uses less memory and is faster.

Previously:
Benchmark_base64Wrap    1000    2177418 ns/op    389576 B/op    2304 allocs/op

Now:
Benchmark_base64Wrap    2000    1282537 ns/op       145 B/op       1 allocs/op
Rewrite quotePrintEncode, add tests.

Problems included:
 - UTF-8 text was broken.  The input string was iterated over as runes,
   but any multi-byte character would have been broken.  E.g., the rune
   U+1f378 should be "=F0=9F=8D=B8".
 - Further, bytes less than 16 were also broken.  E.g.,
     fmt.Printf("=%X", '\r') == "=D"
 - Line breaks were not handled correctly; quoted-printable uses CRLF
   linebreaks.

Though the old approach wasn't entirely correct, it could have been
trivially fixed.  Thus, it's worth comparing benchmarks:

Old:
Benchmark_quotedPrintEncode    10000    125393 ns/op    9520 B/op    598 allocs/op

New:
Benchmark_quotedPrintEncode    500000      5325 ns/op      8 B/op      1 allocs/op
Merge pull request #4 from jeddenlea/master

Merging pull request
- Attachments is now a slice
- Speed/Memory optimizations