Add close method to app

[HansWeltar]

Fixes Issue #370 Session/socket never closed, warnings shown

The msal application does not close it's resources like eg: self.http_client.
As a result a 'ResourceWarning unclosed <ssl.SSLSocket' is shown when the
app goes out of scope (when warnings are enabled)

This fix adds and explicit close method so developpers can cleanly close
the app if they so desire.

[HansWeltar]

A better fix might be needed. A close() method does not help in case of __init__ errors.
For example when you provide a wrong tenant_id

try:
    app = msal.application.ConfidentialClientApplication(APPID, client_credential=SECRET, 
        authority="https://login.microsoftonline.com/" + FAULTY_TENANT)
except Exception:
    app.close() -> fails because app is not defined....

[ghost]

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[HansWeltar]

adressed

A better fix might be needed. A close() method does not help in case of init errors.
For example when you provide a wrong tenant_id

try:
    app = msal.application.ConfidentialClientApplication(APPID, client_credential=SECRET, 
        authority="https://login.microsoftonline.com/" + FAULTY_TENANT)
except Exception:
    app.close() -> fails because app is not defined....

[bgavrilMS]

Seems like the original issue still exists? Is this PR valid?