Support using custom HTTP client

[stevenroose]

Solves #49

[erubboli-bfx]

ACK, good idea

[jayme-github]

What do you think of going one step further adding an interface for HTTPClient so one can use wrapped http.Client's (handling retries, exponential backoff etc.)?
jayme-github@c25e016

[stevenroose]

@jayme-github makes sense. I currently can't work on this as my computer recently got stolen. A separate PR maybe?

[jayme-github]

Let's see what the bfx people say. If they'r okay with it, I can send an additional PR (or you've merged my commit into your PR in the meantime :-))

[stevenroose]

If you can file a PR in my fork from yours, I can. But I can't get into actual Git on this computer right now :p

[jvatianou-connamara]

putting into review/qa for someone from bitfinex to ultimately merge.

thank you for the contribution.

[stevenroose]

It seems to have an ACK from bitfinex for a long time :D

[d8x]

any updates on that topic? Would be great to have that merged :)