Add support for custom header sizes

[aledbf]

fixes #37

[k8s-reviewable]

This change is 

[coveralls]

Coverage increased (+0.02%) to 43.305% when pulling b38afeb4807ab3af884b89553e812cdc7936a497 on aledbf:custom-header-size into fbcedc0 on kubernetes:master.

[aledbf]

I0121 15:55:03.037908       5 nginx.go:210] --- /tmp/a682478286	2017-01-21 15:55:03.000000000 +0000
+++ /tmp/b721520085	2017-01-21 15:55:03.000000000 +0000
@@ -39,7 +39,7 @@

     keepalive_timeout 75s;

-    client_header_buffer_size       1k;
+    client_header_buffer_size       10k;
     large_client_header_buffers     4 8k;

     types_hash_max_size             2048;

[coveralls]

Coverage remained the same at 43.251% when pulling 87322b8 on aledbf:custom-header-size into 30a6229 on kubernetes:master.

[justinsb]

LGTM!