Add ability to bypass disk storage of Credential Store

[epenet]

When used in API mode and inside a long running service, it might be necessary to have multiple Credential Store in parallel.
In this case, the Credential Store shouldn't be storing anything on the disk, and keep everything in memory.

This PR splits the CredentialStore class into three separate classes:

• CredentialStore => no longer writes to disk
• FileCredentialStore => inherits CredentialStore and add storage to disk
• DefaultCredentialStore => replaces the old "static" CredentialStore for use within the pyze client

[jamesremuscat]

The only relevant commit here is 3d65aef, correct?

[epenet]

Yes, you're right. The other two are linked to #61.
Without #61 merged (or an equivalent method) merged, then this one will not work...

[epenet]

New PR #69 created to avoid the leak from PR 61.
I will close this one.